This invention relates to contacts and interconnects for semiconductor integrated circuit devices, and more particularly to methods of manufacture of VLSI devices employing improved metal contacts.
In the manufacture of integrated circuits, aluminum or aluminum alloys are most commonly used for contacts and interconnections, both for single level metallization and multilevel metallization. Aluminum and its alloys enjoy the advantages of low cost, reasonably good conductivity, and good adherence to both silicon and silicon dioxide; also, deposition equipment and techniques, and chemical etchants, are readily available for deposition and selective removal.
However, as integrated circuit geometries shrink and complexity and bar size increase, aluminum-based integrated circuit metallurgy presents serious problems, further aggravated by multilevel metallization and the associated process complexity. Specifically, the problems include deformation at high temperature, thermal expansion mismatch, electromigration and corrosion.
Aluminum, even when doped with copper or titanium, tends to grow hillocks when heated to temperatures of about 400.degree. C. or more. These hillocks can be spires of two microns in height above a one micron film, resulting in unwanted etching where photoresist is either absent or severely thinned over a hillock. This causes open circuits, and can also cause interlevel shorts in multilevel metal devices because a growing hillock can fracture an interlevel insulator (such as silicon oxide) above it if the insulator is thin.
The linear coefficient of expansion of aluminum is not a good match to silicon and is a factor of 50 greater than silicon dioxide. (Al=25.times.10.sup.-6 /.degree.C., SiO=0.5.times.10.sup.-6 .degree.C./Si=4.times.10.sup.-6 /.degree.C.). The expansion mismatch can also fracture an overlying insulator, but more importantly, the expansion of aluminum plays a major role in the failure of large integrated circuits during temperature cycle testing, where the failure mechanism is shorting adjacent leads due to shear forces at the corners of large bars. The source of the stress is apparently the mold compound.
Another problem with aluminum or aluminum alloys is electromigration failures, which are open leads caused by high current density. High current can occur by design or by accidental reduction in the cross sectional area of the lead. Inadequate step coverage or notched or voided leads are examples of how this reduction can occur. Aluminum is one of the metals least resistant to electromigration due to its relatively low melting point.
Also aluminum is corroded by trace amounts of chloride ions. Copper-doped aluminum is even more susceptible to corrosion, so if this alloy is used to increase electromigration resistance, the device becomes more corrosion prone. Plasma etching or reactive ion etching of aluminum, or aluminum alloyed with copper or titanium, requires gaseous chlorine; if the residual chloride contamination is not absolutely controlled, corrosion will occur.
Another problem inherent with aluminum, and more so with copper doped aluminum, is that in multilevel metal technology, the resistivity of the metal dictates the metal thickness for a given design. And with three or more levels of metal the thicknesses are additive. This results in topography of the integrated circuits with peak to valley height differentials of several microns. Any subsequent photoresist processes then are most difficult, because the depth of field for focused exposure radiation is quite different at different topographical heights. Usually, photoresist is overexposed at peaks and underexposed in valleys, resulting in loss of line width control and tendency to electromigration failures. Use of a more conductive material and thinner metallization is desirable to reduce this effect.
Particularly in manufacture of bipolar integrated circuitry using Schottky technology, aluminum reacts with the platinum silicide of Schottky diodes and causes undesirable changes in the electrical characteristics if temperatures of 400.degree. C. are reached. Since 450.degree. C. is a common sinter/anneal temperature in device processing, a barrier layer is interposed, typically Ti:W, to prevent diode and contact degradation. The diffusivity of aluminum through this barrier layer dictates a thickness, which adds to the thickness of the composite metal film and complicates multilevel processing.
A method of overcoming the foregoing problems is disclosed in our copending application Ser. No. 06/732,547 referred to above. This method involves, in a preferred embodiment plating a selected portion of a metal layer with electroless copper. The method of using a positive photoresist on the metal layer to serve as a plating mask is inadequate as the caustic plating bath dissolves the photoresist. Negative photoresists work but are unable to achieve the very fine geometries required in advanced VLSI devices. Moreover, during stripping of the negative resist the copper oxidizes and makes it difficult to selectively deposit metals such as tungsten thereon.
It is therefore the principal object of this invention to provide an improved method of plating copper onto a selected portion of a metal surface in high-density integrated circuits.